1. Field of the Invention
The present invention relates to a telephone speech recognition system and particularly, a telephone speech recognition system for use with a telephone line, network, or switchboard.
2. Description of the Related Art
A conventional speech recognition system connected to a telephone line is now explained referring to FIG. 3. When the speech of a caller is received from a telephone line and has to be identified, it is transmitted via a telephone line interface 31 to the speech recognition system. The conventional speech recognition system substantially comprises an acoustic analyzer 32, a speech pattern matcher 33, and a reference speech model storage 34.
The speech of the caller introduced from the telephone line interface 31 is first fed to the acoustic analyzer 32. In the acoustic analyzer 32, the speech is divided at equal intervals of substantially 10 milliseconds (ms) on the basis of a Hamming window of about 25 ms and subjected to acoustic analysis such as cepstrum analysis to produce an acoustic vector train which is then supplied to the speech pattern matcher 33. The reference speech model storage 34 saves speech models such as HMMs (hidden Markov models). The speech pattern matcher 33 collates the acoustic vector train with the speech models saved in the reference speech model storage 34. Consequently, a succession of symbols representing the speech models at highest likelihood are released as outputs of the speech recognition.
In the conventional manner, the speech recognition is however carried out regardless of the conditions of line connection from a caller at the other end of the line to the telephone line interface 31 at the entrance of the speech recognition system. More specifically, the collating action of the speech pattern matcher 33 with the speech models saved in the reference speech model storage 34 is executed without concerning any of the conditions of line connection. Accordingly, when particular noises from the telephone line are involved or there is a difference in the frequency characteristic between the conventional telephone line and a line used with a system such as a mobile telephone set, a desired level of speech recognition will hardly be accomplished.
It is particularly difficult for such a conventional speech recognition system to recognize voice data in a call received from the international telephone line which varies depending on terminal and line systems of a country.